1. Field of the Invention
The present invention relates to a system for the preventive maintenance of the Photomultiplier Detector Blocks of PET Scanners. This system is used to automatically indicate the expected date of failure of a photomultiplier detector block and provide time for its replacement and maintenance in a scheduled preventive maintenance program, thus eliminating expensive unscheduled downtime of a PET Scanner due to photomultiplier failure.
2. Background of the Related Art
Positron Emission Tomography (PET) scanners typically contain several hundred photomultipliers (PMTs) usually arranged in blocks of four (4) PMTs for detecting gamma radiation. A PMT is an electronic amplifier with an extremely high gain; they are used in PET scanners to amplify a signal up to 100 million times. Photomultiplier tubes are not noted for great stability, and due to its construction characteristics, as it ages the gain of a PMT drifts in time. Therefore, each PMT requires periodic calibration See, Burle Industries Inc., "Photomultiplier Handbook", TP-136, 9 (1989); and Hayashi, "Use of Photomultiplier Tubes in Scintillation Applications", Application Res-0790, Chap. 111-7: Pulse Height Stability, page 5, Hamamatsu TV Co., LTD. (1990).
During the use of a PET scanner the magnitude of gain fluctuations of these photomultipliers can become so large that the photomultipliers cannot be calibrated, disabling the scanner until the detector blocks containing the defective photomultipliers are replaced. PET scanner manufacturers attempt to avoid photomultiplier failure by selecting photomultipliers which successfully complete an aging process over several hours, called burn-in testing. This testing, however, cannot predict the behavior of these photomultipliers months or years after being put into service. To account for the aging of the photomultipliers, the PET scanner must be periodically recalibrated, typically once a week by iteratively attenuating the gain of the photomultipliers in each detector block.
Accordingly, in the sale of a PET scanner and its host computer system, each company provides a gain control program to allow automatic adjustment of the PMT gain in each detector block. This type of computer program is routinely provided to the buyer, along with many other software programs required for the correct operation of the PET scanner. Although the PMT calibration is a basic necessity of the PET scanner, and the information is routinely available to the user, none of the manufacturers provide an automatic system for the preventative maintenance of the PMT detector blocks. Rather, all of the PET scanner manufacturers replace a PMT detector block only after it has failed. Since the cost of a typical PET scanner ranges from more than one million ($1,000,000.00) to over two million ($2,000,000.00) dollars, manufacturers are aware of the value to their customers of purchasing a highly reliable PET scanner.
The following manufacturers utilize several hundred PMTs in each of their PET scanner models listed below. All of these PET scanners require periodic calibration of their PMT gains by the user, utilizing the control program supplied with the host computer system that controls the scanner. Therefore, each would greatly benefit from an automatic preventative maintenance system.
SIEMENS MEDICAL SYSTEMS
Location: 111 Northfield Ave., West Orange, N.J., 07052 PA0 PET Scanner CTI-931, CTI-951, CTI-953, EXACT 31, PA0 Models: EXACT 47 PA0 Cost: From $2,200,000.00 to $2,600,000.00 PA0 Location: P.O. Box 414, Milwaukee, Wis., 53201 PA0 PET Scanner PA0 Models: PC-384, PC-1024, PC-2048, PC-4096 PA0 Cost: From $2,000,000.00 to $2,500,000.00 PA0 Location: P.O. Box 6910, 360 Foothill Road, Bridgewater, N.J., 08807 PA0 PET Scanner PA0 Model: SHR-5000 (for animals only) PA0 Cost: $1,100,000.00 PA0 Location: 3401 Market Street, Suite 272, Philadelphia, Pa., 19104 PA0 PET Scanner PA0 Model: PENN-PET-300-H PA0 Cost: $1,300,000.00
GENERAL ELECTRIC MEDICAL SYSTEMS
PHOTONICS RESEARCH CORPORATION (HAMAMATSU)
UGM MEDICAL SYSTEMS
During the useful lifetime of a PET scanner, one or more detector blocks will unexpectedly reach their limit of calibration range causing failure of the PET scanner. Since all of the above PET scanner manufacturers presently replace PMT blocks only after they have reached their limit of calibration range and have failed, the scanner must be turned off for one to two days. The user must typically wait for the repair technician to arrive, change the defective PMT blocks and recalibrate the scanner with the newly installed PMT blocks.
The unexpected loss of a PET scanner due to failure of a PMT block is very costly. In a hospital environment, which performs fast routine clinical tests, patient cancellations could amount to over $8,000.00 per day. In a research environment, such as BNL or a University, the cost of an unexpected PET scanner failure is even higher. Failure of the scanner may result in the cancellation of serial planned studies requiring the preparation of expensive and highly perishable medications. Cancellation may also disrupt experiments conducted under very stringent research protocols that monitor the development over time of an experimental treatment, which can not be easily duplicated. The loss of funds for research personnel and research equipment left idle by the scanner's failure, such as the associated research cyclotron and the chemists who prepare the experimental medications with an attached tracer isotope, is extremely high and can cause the budget allocated to a research project to be exceeded.
Accordingly, it would be desirable to provide a system for predicting when photomultiplier detector blocks should be routinely replaced prior to reaching their limit of calibration range, thus avoiding interruption of the use of the PET scanner due to failure of photomultiplier detector blocks.